This invention relates to a status check method for a data transmission system in which a plurality of terminal units are mutually connected by a first and a second loop transmission line for transmitting data in a first direction and a second direction which is opposite to said first direction, respectively.
The data transmission system has such a very simple structure that basically a transmission line is arranged in the form of a loop, at arbitrary positions of which nodes (a kind of switch for a data transmission control unit) are provided. Since such a data transmission system often has node and line troubles, a data transmission system having dual loop transmission line as shown in FIG. 1 has been developed, recently. In such a system, in order that, even if both loop transmission lines 1 and 2 are broken at a point between the nodes N.sub.1, N.sub.2, N.sub.3 and N.sub.4 of the same structure, the whole system may not break down, the data transmission of the loop transmission lines 1 and 2 are executed in the respective directions which are opposite to each other. That is, the nodes N.sub.1, N.sub.2, N.sub.3, and N.sub.4 have a loop-back function. For example, even when the loop transmission lines 1 and 2 are separated at a position denoted by X, data can still be circulated by connecting two loop transmission lines at N.sub.1 and N.sub.4 which are adjacent to the broken point. The trouble point detection that should be done before the above-mentioned loop-back operation has been made as follows. That is, a transmission control unit which forms a loop node sends out a loop check message for specifying an address of loop-back, and it is checked whether this message returns. Further, in a system of the prior applications of the present applicants (the Japanese patent application No. 115300/79 and the corresponding foreign patent applications of U.S. Ser. No. 149,632, Germany Application No. P3020077.2 and France Application No. 8011986) where transmission control units are connected in the respective dual loop transmission lines and further bypass transmission lines are provided between the corresponding pairs of the transmission control units, each transmission control unit sends out a loop check message attached with an address thereof. It is checked whether the message makes a round on a minor loop formed by two adjacent pairs of transmission control units (hereinafter referred to as a minor loop check).
Both of the above-mentioned trouble detection methods require insertion of an address in the check message. However, when a new node is added to the loop transmission line, or when its own stored address of the transmission control unit at a node is vanished after recovery from a trouble, it may happen that the node address is unknown. In such a case, prior to the abnormality detection of the system, it is necessary to investigate the address status of other nodes in the system and to set a new node address manually such that it is not equal to those of the other nodes.
Therefore, the above-mentioned two trouble detection methods are defective in that loop check is not possible before the addresses of the added and recovered nodes are set.